EP0527739A1

EP0527739A1 - Servo guided stage system.

Info

Publication number: EP0527739A1
Application number: EP91902842A
Authority: EP
Inventors: Samuel Kay Doran; Robert Arthur Kendall
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1990-04-30
Filing date: 1990-10-12
Publication date: 1993-02-24
Anticipated expiration: 2010-10-12
Also published as: US5140242A; EP0527739B1; DE69027335D1; WO1991017564A1; DE69027335T2; JPH05504731A; JPH07106516B2

Abstract

Un étage de positionnement comporte une base, une plaque montée pour glisser sur ladite base, et au moins trois dispositifs d'entraînement linéaire destinés à contacter en rotation à la fois la plaque et la base afin de déplacer la plaque sur la surface de la base. Grâce à au moins trois déplacements d'entraînement séparés, la plaque se déplace par rapport à la base selon les axes x et y agencés de manière rectiligne, et la rotation est également prévue. Les dispositifs d'entraînement linéaire comprennent un moteur et un cabestan entraîné par ce moteur, ainsi qu'une barre d'entraînement qui se frotte contre le cabestan. De préférence, l'étage comporte un mécanisme à ressort communiquant une force orientée vers le bas à l'extrémité de ladite barre qui est reliée à ladite plaque par l'intermédiaire d'un joint à rotule. Cette force demeure constante quelle que soit l'extension de la barre. Un joint à rotule permet une modification dynamique de la précharge entre les cônes et la rotule du joint à rotule et jusqu'à des niveaux voulus. Le système de positionnement comprend un système interféromètre constitué d'une structure intégrée possédant une pluralité de systèmes optiques d'interféromètre disposés sur un élément unitaire. Celui-ci comprend des interféromètres intégrés et parallèles à miroir plan et à deux axes.A positioning stage comprises a base, a plate mounted to slide on said base, and at least three linear drive devices intended to contact in rotation both the plate and the base in order to move the plate on the surface of the base . Thanks to at least three separate drive movements, the plate moves relative to the base along the x and y axes arranged in a rectilinear manner, and rotation is also provided. Linear drives include a motor and a capstan driven by this motor, as well as a drive bar which rubs against the capstan. Preferably, the stage comprises a spring mechanism communicating a force oriented downwards at the end of said bar which is connected to said plate by means of a ball joint. This force remains constant whatever the extension of the bar. A ball joint allows dynamic modification of the preload between the cones and the ball joint of the ball joint and up to desired levels. The positioning system comprises an interferometer system consisting of an integrated structure having a plurality of optical interferometer systems arranged on a unitary element. This includes integrated and parallel interferometers with plane mirror and two axes.

Description

SERVO GUIDED STAGE SYSTEM

Background of the Invention

1) Field of the Invention

This invention is directed to a drive mechanism for accurately positioning a work stage along two axes with three degrees of freedom.

CROSS REFERENCE TO RELATED APPLICATION

U.S. application Serial No. entitled

"Two Axis Plane Mirror Interferometer", filed contemporaneously herewith, by R.A. Kendall and assigned to the assignee of this application, and incorporated herein by reference.

U.S. application Serial No. entitled

"Ball Joint with Dynamic Preload Adjustment", filed contemporaneously herewith, by R.A. Kendall and assigned to the assignee of this application, and incorporated herein by reference.

2) Related Art

United States patent No. 4,528,490 of Hansen for "Two Axis Drive for Stage" includes a base plate and an additional pair of stacked plates, one known as a stage plate and the other as an intermediate plate (20) . Each plate is driven by a drive. The intermediate plate (20) is driven by drive bar ⁽42⁾ along a linear direction with respect to the base plate. A drive means for the stage plate includes a motor driven capstan, and a drive bar (66⁾ has angular freedom of movement, i.e. is pivotally mounted under stage plate (12) . The stage plate (12⁾ is thus free to move along a second path and to rotate.

J. Reed et al "High Speed Precision X-Y Stage", J. Vac. Sci. Technol. B, Vol. 3, No. 1 ppll2 et seq., (Jan/Feb 1985) describes conventional linear ways and ball bearings but "incorporates a unique capstan/swinging drive bar design to couple the servomotors and the X and Y stage elements." The stage involves three stacked plates coupled to one another by crossed linear bearings. The bottom plate is affixed to a base. The center plate is fixed rigidly to its drive bar through a preloaded duplex bearing (FIG. 2) . The drive bars are driven by a servomotor via a friction-drive capstan. s. Ido et al "Precision X-Y Stage for Electron

Beam Lithography System" pp 267-268 Bull. Japan Soc. of Prec. Eng.., Vol. 18, No. 3 (Sept. 1984) describes a stacked X-Y stage configuration with hydrostatic bearings. Kallmayer et al "X-Y Table" IBM Technical

Disclosure Bulletin Vol. 30, No. 7 (Dec. 1987) , pp. 376-377 shows three rigidly mounted spindle drives with stators 1, 2, and 3 affixed to a rigid support so they do not pivot relative to the table 4, so the flexibility of the drives is limited by their rigid mounting to a restricted range of motions afforded by the guides 10 and 11 in the slots in the table 4. In addition, two of the spindle drives 1 and 2 are parallel to each other. Laser interferometers referred to in that article as 5, 6 and 7 and mirrors 8 and 9 are used to measure displacement.

Tsuyuzaki et al J. Vac. Sci. Technol. B, Vol. 4, No. 1 p28X, (Jan/Feb 1986) describes a plate structure or "planar" stage with X-Y motion accuracy imparted via machined guide slots in the base and substrate positioning table. An X-Y cross structure is placed within these slots between the top table and the base. A low friction polymeric material such as PTFE is employed as a bearing surface. Constant Download Friction Drive System, IBM Technical Disclosure Bulletin, Vol. 32, No. 8A, (Jan.

1990) pages 120-121 describes a method and apparatus for cancelling the variation in Download exerted by the end of a drivebar system by preloading the drivebar as shown in FIG 3 herein.

Ball Joint Pivot with Dynamic Preload Discussion of Related Art

Ball type pivot joints provide multiple degrees of freedom about a point at the center of a ball of spherical shape. Heretofore, ball joints have included two basic types including as follows:

(1) Spring-loaded joints and

(2) Spherical bearing.

Spring-loaded ball joints include a pair of cones or sockets which are tightened against the ball to eliminate backlash.

A disadvantage of that system is that is that when using high preloads for high linear stiffness, there is considerable friction and wear between the ball and the cones or sockets.

The spherical bearing ball joint relies on closely matched inner and outer spherical bearing elements. This type of ball joint has high linear stiffness and low friction, but by the nature of this design must operate with a small clearance between the inner and outer bearings. Such a small clearance increases with wear and the backlash associated with this clearance makes it unsuitable for micropositioning applications. Summary of the Invention

A positioning stage comprises a) a base; b) a stage plate slideably supported on said base, c) at least three linear drive means rotatably engaging both the stage plate and rotatably engaging the base for moving the stage plate on a path with at least three separate drive displacements along the rectilinearly disposed x and y axes and rotation of the plate on the base substantially parallel to the surface of the base, the plate moving upon the surface of the base, whereby combined x, y rectilinear and rotary motions with respect to the base can be achieved. The positioning system includes an interferometer system including an integrated structure including a plurality of interferometer optical systems on a unitary element the system including integrated, parallel, dual two axis plane mirror interferometers in the unitary element.

Preferably the stage includes as follows: c a) a first one of the linear drive means is directed in a first direction substantially along the x axis, b) a second one of the linear drive means is directed substantially along the y axis transverse to the direction of the first linear drive means, and c) a third one of the linear drive means is substantially parallel to one of the first and second linear drive means for providing rotation through the angle theta, whereby displacement of the stage can be provided in the x, y and rotational directions of motion relative to the base as the three linear drive means independently operate to translate the stage through linear and/or rotary motion with respect to the base.

In still another aspect the stage preferably includes as follows: a) each of drive means on the base being connected to the stage plate for moving the stage plate as a function of its force, b) the drive means including a motor and a capstan driven by the motor, c) a drivebar and pressure means, d) the capstan and the pressure means cooperating to drive the drivebar in response to the motor, e) the drivebars being pivotally mounted on the stage plate and frictionally engaged with the capstan by the pressure means; whereby rotation of the capstan moves the stage plate along a path as a function of the at least three displacements provided by the independent drive means .

In another aspect, the stage includes a) each of drive means on the base being connected to the stage plate for moving the stage plate as a unction of its force, b) the drive means including a motor and a capstan driven by the motor, c) a drivebar and pressure means, d) the capstan and the pressure means cooperating to drive the drivebar in response to the motor, e) the drivebars being pivotally mounted on the stage plate and frictionally engaged with the capstan by the pressure means; whereby rotation of the capstan moves the stage plate along a path as a function of the at least three displacements provided by the independent drive means. In another aspect of the invention, a positioning system for a stage comprises: a) a base; b) a stage plate slideably supported on the base, c) at least three linear drive means rotatably engaging the stage plate and rotatably engaging the base for moving the stage plate on a path with at least three separate drive displacements motions upon the surface of the base for moving the plate upon the surface of the base, d) means for detecting the displacement of each of the linear drive means, e) control electronics means for providing control signals having inputs connected to the means for detecting and having outputs connected to the linear drive means. whereby combined rectilinear and rotary motion with respect to the base can be achieved. In the past the x, y drives for supporting work to be machined have been limited to the use of stacked stages for x and y displacement with one on each level of the stack. A guide track is required for each level. A serious problem of alignment of the tracks and bearing adjustment limits the long term accuracy and serviceability of such a system.

In addition with more demanding tolerances with decreasing size of chips, etc., the effects of thermal coefficients of expansion are becoming more and more a limiting factor. Rotation through a substantial angle theta on the surface of the base carrying the stage is also a problem for these systems. Such rotation is required where the rails or the bearings of prior art systems cause misalignment.

A positioning stage includes a base, a stage plate slideably supported on the base, at least three linear drives rotatably engaging both the stage plate, and the base for moving the stage plate on a path with at least three separate drive displacements parallel to the surface of the base, with the plate moving upon the surface of the base. This stage provides combined rectilinear and rotary motion with respect to the base.

Preferably, a first one of the linear drive means is directed in a first direction, a second one of the linear drive means is directed substantially transverse to the direction of the first linear drive means and a third one of the linear drive means is substantially parallel to one of the first and second linear drive means. In this way, displacement of the stage can be provided in the x, y and rotational directions of motion relative to the base, as the three linear drive means independently operate to translate the stage through linear and/or rotary motion with respect to the base.

A ball joint in accordance with a preferred embodiment of this invention provides the ability to alter the preload between the cones and the ball of the ball joint dynamically to desired levels. This is a very significant advantage for use in micro-positioning applications such as an X-Y stepper stage. When the stage or other device is in motion, the preload can be reduced to provide low friction and wear. When the stage or other device is not in motion, then the preload can be increased to provide zero backlash and high stiffness.

A ball-type pivot joint with dynamic preload adjustment includes a ball and a pair of cones which are dynamically preloaded by servo (feedback) control of a preload actuator. Alternate designs are limited by manufacturing and assembly tolerances as well as wear of bearing surfaces or are subject to change due to thermal fluctuations.

Brief Description of the Drawings

FIG. 1 is a perspective view of an x,y, theta stage plate and a plurality of linear drives including drivebars driven by friction drive units and a control system for positioning the stage plate.

FIG. 2 shows an example of a prior art drivebar system.

FIG. 3 shows a preload roller which has been added to the drivebar system of FIG. 2. FIG. 4 shows a detailed plan view of a capstan and pinch rollers for driving a drivebar in a first diagonal position.

FIG. 5 is a variation of FIG. 4 in which the drivebar is shown in a vertical position after rotation of the capstan and pinch rollers about the axis of the capstan.

FIG. 6 is a further variation of the drive of

FIG. 5 where the drivebar has been rotated to the opposite diagonal position. FIGS. 7-10 show plan views of the stage of FIG.

1 which have been simplified for the purpose of illustrating the way in which drivebars in accordance with this invention can be driven linearly to locate the stage plate anywhere on the base supporting it. FIG. 10 also shows rotation of the stage plate about an angle Theta.

FIGS. 11, 12 and 13 show three alternative arrangements for a ball joint in accordance with this invention with conical bearings therefor for use in the linkage between one of the drivebars and the stage plate.

FIG. 11 shows a partially sectional view of a ball joint linkage with an automatically controlled actuator for preloading the ball joint. FIG. 12 is a modification of the ball joint linkage of FIG. 11.

FIG. 13 is another embodiment of the ball joint linkage analogous to FIG. 12 but the structure for applying force to the cones and the ball in the joint is modified.

FIG. 14 shows the electrical schematic diagram of the control system for the drives employed to position the stage of FIG. 1.

FIGS. 15A-15C show bottom, front and right side views of a pair of interferometers of the kind shown in FIG. 1 and 2 for the Y and Theta motion of the stage.

FIGS. 16A-16C show modifications of the interferometers of FIGS. 15A-15C. FIGS. 17A-17C show further modifications of the interferometers of FIGS. 15A-15C.

Description of the Preferred Embodiment

Referring to FIG. 1, a base 10 supports a stage plate 11, adapted for carrying a work piece 8 slideably supported for moving on the upper surface of the base 10. The plate moves along the rectilinear x and y axes which rest substantially parallel with the flat surface of base 10. Base 10 is preferably a very flat, massive stable table composed of a material such as granite, ceramic or steel with a highly polished, extremely flat planar upper surface 9 which carries the X-Y stage plate 11, with plate 11 supporting on its upper surface a work piece 8. The lower surface of stage plate 11 is slideably supported on the upper surface 9 of base 10 by low friction supports such as feet 12 composed of a low friction polymeric material such as PTFE

(polytetrafluoroethylene) or, alternatively, the feet

12 can be replaced by equivalent support bearings such as air bearings or roller bearings.

To the X-Y stage plate 11 there are pivotably secured three linear, friction drive units XI, Yl and

Y2. The drive unit XI includes a drive motor Ml which is located along one side of base 10 and the Yl and Y2 include drive motors M2 and M3 which are located along the adjacent side of base 10 spaced well apart from each other to provide three degrees of freedom to the stage plate 11. The transmission of power for drive unit XI on one side of the Xl-drivebar 16 (composed of ceramic or steel material) is provided by two pinch rollers 13 and 14 and a capstan 15 driven by motor Ml. The XI drive capstan 15 is located on the opposite side of

Xl-drivebar 16 from pinch rollers 13 and 14 so that a friction drive is provided by the capstan 15 and the two rollers 13 and 14. Preload roller 20 presses down on the top of Xl-drivebar 16. Pinch rollers 13 and 14 as well as drive capstan 15 and roller 20 are mounted on carriage 18 to rotate about pivot PI along the axis of the shaft of the motor of drive unit XI to permit pivoting of drivebar 16. The rollers 13 and 14 on one side and the capstan 15 on the other side exert opposing forces which act together to provide friction drive engagement of the capstan 15 with drivebar 16 for reciprocating it longitudinally as capstan 15 turns while concomitantly permitting the drivebar 16 and the carriage 18 carried on the shaft of capstan 15 and rollers 13 and 14 to pivot about the axis PI of the shaft of the capstan 15, thereby permitting rotation of the drivebar 16 on carriage 18 and the shaft of capstan 15. Drivebar 16 is offset at its inner end 17 where it connects to a linkage including pin 19 secured to stage plate 11 to secure the drivebar 16 to stage plate 11. The position of the stage plate 11 is measured by a laser interferometer system with a pair of bars 50 and 51 secured to two orthogonal sides of plate 11 opposite from the drive units XI, Yl and Y2. Each of bars 50 and 51 has a mirrored surface 73 and 52 respectively for measuring the X axis and Y axis displacements of stage plate 11. Laser beam 76 is provided to interferometer 79 which produces beams 77, 78, 80 and 81 which emanate from interferometer 79 towards mirror 52 and which are reflected back from mirror 52 to the interferometers 79 which reduces output beams 82 and 83 which pass to receivers 84 and 85 from the interferometers 79. Receivers 84 and 85 as well as receiver 45 are optical-to-electrical transducers for converting the laser signal to electronic signals. The receivers

-11- 84, 85 and 45 include a lens which focuses the laser beam onto an active chip of a silicon photodiode.

Each receiver (which can be a commercially available product such as the Hewlett Packard 10780A receiver) , includes a photodetector, an amplifier and level translator, a line driver, a level sensor

(comparator) and local voltage regulators. The receivers 84, 85 and 45 convert the Doppler-shifted laser light into electrical signals that can be processed by the electronic system. This is done to determine the position of mirror 52 along the Y axis, measured from the opposite side of the plate 11 from the Yl and Y2 drive units. Beam 75 passes through interferometer 110 producing beams 111 and 112 which measure the X axis position of a point on mirror 73 by reflection of beams 111 and 112 from mirror 73 to interferometer 110 which produces an output beam 46 directed at receiver 45.

Theta Angle Measurement Apparatus With respect to measurement of Theta motion by the laser interferometer system, within a small range of angles sufficient for applications such as VLSI semiconductor chip manufacture, the interferometer apertures are wide enough to receive the beams as the angles of reflection change. For greater angles of rotation, the system can employ a system of markings upon the drivebars which will give absolute positions of the drivebars with respect to their linear drives.

Constant Download Friction Drive System FIGS. 2 and 3 illustrate how the variation in download exerted by the ends of drivebars 16, 26 and

36 is minimized.

Friction drive units are frequently used in mechanical systems for highly precise positioning of components such as work pieces and the like. A problem with such systems is that the download of the drivebar upon the driven component varies with the cantilevered extension of the drivebar along its path. FIG. 2 shows an example of a prior art drivebar system. If a drivebar 16 of length L and weight W is extended by a distance B beyond where it is supported by a friction drive unit X with a capstan 15" contacting drivebar 16 at the drive point, it will exert a force of W(B-L/2) /B on the driven object: stage plate 11. The stage plate 11 is shown supported on rollers 12' in this modified embodiment. It is assumed that the center of gravity of the drivebar 16 is at L/2, but this is not necessary. The variation of the download force causes a variation in the strain or distorting of the driven object. If the object is driven off center, a varying pitching moment is also applied to the load. It should be noted that a download force at the end of the drivebar 16 is not necessarily a disadvantage because it can be used as a preload. It is the variation in the download force as a function of the extension of the drivebar 16 which causes the problem. FIG. 3 shows a preload roller 20' which is added to the drivebar system of FIG. 2. The preload-roller 20' under the force of a spring 20' ' exerts a force P preloading the drivebar 16 at a distance C from the drive point and exerts a force PC/B on the driven object. (While a spring 20 ' ' is preferably employed as shown in FIG. 3, in the embodiment of FIG. 1 the spring has been omitted from FIG. 1, for convenience of illustration.) If the product PC equals WL/2, then the download force at the distal end of drivebar 16 where it connects to the driven object: stage plate 11 does not change materially as the drivebar

16 extends and retracts. The product PC is a torque exerted about the drive point. Means other than a preload-roller 20' can be used to create this torque.

FIGS. 4, 5 and 6 are simplified illustrations of the rotatable carriages 28' (similar to carriages 18,

28 and 38 in FIG. 1) carrying elements of a friction drive adapted to produce linear motion of one of the drivebars 16, 26, and 36 showing one of the rotatable drive capstans 15, 25 and 35. In this case capstan

25, which is turned by motor M2, is holding the drivebar 26 between the side of capstan 25 and the sides of pinch rollers 23 and 24. The capstan 25 is supported on a shaft (not shown) which has a bearing within carriage 28' which has been modified in that the preload roller 30 has been omitted for ease of illustration. Carriage 28' rotates about the shaft carrying capstan 25 so that the drivebar 26 can rotate from the position in FIG. 4 where it is rotated from diagonal orientation to a vertical position in FIG. 5 to a reverse diagonal position in

FIG. 6. It should be noted that for the Xl-drivebar

16, FIG. 4 is analogous to FIG. 9, FIG. 5 is analogous to FIG. 7 and FIG. 6 is analogous to FIG^'. 8.

X, Y, Theta, Three-Linear-Drivebar System

Xl-drivebar 16 reciprocates in general in parallel with the X axis as indicated in FIG. 1, with rotation about PI axis away from parallel with the x axis to afford enhanced flexibility of being able to provide positioning of pin 19 and stage plate 11 anywhere within predetermined boundaries of base 10.

As can be seen in FIGS. 7-10, the plate 11 can be rotated through an angle theta (as shown in FIG. 10) with respect to the X and Y axes using the three -14- drive assemblies of FIG. 1 in cooperation, where the displacement of drivebars 26 and 36 is unequal.

FIGS. 7-10 show plan views of the stage of FIG.

1 which have been simplified for the purpose of illustrating the way in which drivebars 16, 26 and 36 in accordance with this invention can be driven linearly to locate the stage plate 11 anywhere on the base 9 supporting it.

FIGS. 7-9 illustrate the capacity of the drive to position the stage plate 11 in the four remote positions on the base 10. FIG. 10 shows rotation of the stage plate 11 about an angle Theta.

FIGS. 7-10 show plan views of the table of FIG. 1 which have been simplified for the purpose of illustrating the way in which the three drivebars 16, 26 and 36 can be driven linearly to locate the stage plate 11 anywhere on the base 10. The work piece 8 shown represents a wafer with chips indicated by squares on the work piece 8. Rotation through angle Theta is shown by FIG. 10.

In FIG. 7, all the drivebars 16, 26 and 36 are approximately half way extended with the Xl-drivebar 16 horizontally oriented, parallel to the X axis, and the Y-drivebars 26 and 36 vertically oriented parallel to the Y axis.

FIGS. 8 and 9 illustrate movement of plate 11 and work piece 8 to two opposite corners of the base 10 and it is obvious that the other two corners can be reached by analogous operation of the drivebars. To move the plate 11 to the left, the Xl-drivebar 16 is driven to the left. To move plate 11 to the right x-1 drivebar 16 is driven to the right. In each case, the drivebars 26 and 36 will pivot about the pivots P2 and P3. In FIG. 8, drive unit XI and drivebar 16 have been rotated clockwise. Drive units Yl and Y2 as well as drivebars 26 and 36 have been rotated counter-clockwise. All three drivebars 16, 26 and 36 have been extended to their fullest extension to move the stage plate 11 to its distal position, i.e. farthest from the three drive units XI, Yl , and Y2.

In addition, to move the plate 11 to the upper left, drivebar 16 is driven to the left and drivebars

26 and 36 are driven upwardly, with the drivebar 16 rotating clockwise about axis PI, and drivebars 26 and 36 rotating counter-clockwise about pivots P2 and

P3, as shown in FIG. 8.

To move the plate 11 to the lower right of the base 10 as shown in FIG. 9, the drivebars 26 and 36 are retracted downwardly to near their lowest excursion with the drivebar 16 rotated counter-clockwise from its positions in FIGS. 7 and

8, and with drivebars 26 and 36 rotated to their clockwise extreme positions. To move to the lower left of base 10, the drivebars 26 and 36 will be rotated to their counter-clockwise extremes as the drivebar 16 is moved to the left along the X-axis, rotating slightly clockwise to move the plate 11 to its proximal position. In FIG. 9, drive unit XI and drivebar 16 have been rotated counter-clockwise. Drive units Yl and

Y2 as well as drivebars 26 and 36 have been rotated clockwise. all three drivebars 16, 26 and 36 have been retracted to their shortest extension to move the stage plate 11 to its proximal position, i.e. nearest to the three drive units XI, Yl , and Y2.

In FIG. 10 the rotation from the position in

FIG. 7 has been achieved by driving the second Y-axis drive 36 downwardly while holding the first Y-axis drivebar 26 and the Xl-axis drivebar 26 stationary. Pinch rollers 23 and 24 cooperate with capstan

25 to provide friction drive engagement with drivebar

26 which is secured at inner end 27 to pin 29 which connects the drivebar 26 to stage plate 11. Pinch rollers 23 and 24 as well as drive capstan 25 and roller 30 are mounted on carriage 28 to rotate about pivot P2 along the axis of the shaft of the motor M2 of drive unit Y2.

Pinch rollers 33, 34 cooperate respectively with capstan 35 to engage with drivebar 36 which is secured at inner end 37 to pin 39 which connects the drive unit Y2 to stage plate 11. Pinch rollers 33 and 34 as well as drive capstan 35 and roller 40 are mounted on carriage 38 to rotate about pivot P3 along the axis of the shaft of the motor of drive unit Yl .

Ball Joint Pivot with Dynamic Preload A ball-type pivot joint in accordance with this invention employs dynamic preload adjustment of the ball joint. The ball joint includes a ball and a pair of cones which are dynamically preloaded by servo (feedback) control of a preload actuator. Alternate designs are limited by manufacturing and assembly tolerances as well as wear of bearing^' surfaces or are subject to change due to thermal fluctuations.

Ball type pivot joints provide multiple degrees of freedom about a point at the center of a ball of spherical shape. Heretofore, ball joints have included two basic types including spring loaded joints and spherical bearings.

Spring loaded ball joints include a pair of cones or sockets in which the springs press the cones or sockets against the ball to eliminate backlash. A disadvantage of that system is that is that when using high preloads for high linear stiffness, there is considerable friction and wear between the ball and the cones or sockets.

The spherical bearing ball joint relies on closely matched inner and outer spherical bearing elements. This type of ball joint has high linear stiffness and low friction, but by the nature of its design must operate with a small clearance between the inner and outer bearings. Such a small clearance increases with wear and the backlash associated with this clearance makes it unsuitable for micropositioning applications.

The joint in accordance with a preferred embodiment of this invention provides the ability to alter the preload between the cones and the ball dynamically to desired levels. This is a very significant advantage for use in micro-positioning applications such as an X-Y stepper stage. When the stage or other device is in motion, the preload can be reduced to provide low friction and wear. When the stage or other device is not in motion, then the preload can be increased to provide zero backlash and high stiffness.

This is achieved using the arrangement depicted in FIGS. 11-13. Referring to FIG. 11, a drive rod 51

(having a round cross section at the end) with a ball end 50 is located between two cones or sockets 67 and

68 in a fixed member 52 and a preload bar 53, respectively. The cone 67, shown in the preferred embodiment in FIG. 11 as a fixed cone 67, is affixed to an object to be moved. The preload cone 68 is formed in the lower surface of preload bar 53.

Preload bar 53 is connected to flange 64, i.e. pivot end 64. The opposite ends of bar 53 are the flange 64 secured to base 63 and the main body 53 which are flexibly connected together by flexure strip 54 formed between the elongated pair of transverse slots

154 and 254. In summary, flexure strip 54 provides from an elevational point of view a flexure "point" formed between body 53 and flange 64 which forms the pivot end 64 of the preload bar 53.

Flange 64 is affixed to base 63 of fixed member 52 by threaded fasteners 65. Base 63 is a mounting plate having holes therethrough for fastening to the stage plate 11 by threaded fasteners (not shown) .) The flexure pivot 54 permits preload cone 68 in member 53 to be moved through a small angle abut the flexure pivot 54. Preload actuator 55 is mounted so as to provide a compressive preload force acting to press down against the preload member 53 and preload cone 68. Preload actuator 55 comprises a piezoelectric or equivalent actuator such as a pneumatic, hydraulic, voice coil actuator, or the equivalent. The compressive preload force applied to the cones 67 and 68 and to the ball 50 by using a preload tensioning rod 57 and a preload tensioning nut 59 through a hole in the center of preload actuator 55. In addition rod 57 passes through a hole 70 in preload member 53 through the center of the preload cone 68, through ball 50 and through the fixed member 52 and the center of fixed cone 67 as well as a hole 74 in the center of a preload sensor 56 comprising a strain gauge or the equivalent such as a force gauge. Load sensor 56 is mounted so as to detect the load on the tensioning rod 57. Line 62 connects the strain gauge output into the input of the preload servo and drive electronics 58 whose output is connected to the input of the piezoelectric preload actuator 55. To increase the preload force upon the ball joint, the actuator 55 is extended or increases in thickness to compress the ball joint. To reduce the preload force, the actuator is retracted or made thinner, with the servo 58 having a predetermined level programmed into it electronically. The preload force can be altered at a frequency limited only by the bandwidth of the actuator 55. Servo 58 has another input 61 from a system controller for the purpose of setting the desired level of preload. Although FIG. 11 shows a system using a tensioning rod 57 and a closed-loop servo system, other arrangements can be employed to provide dynamic preload adjustment forces on a ball-type pivot joint.

FIG. 12 shows a modified embodiment of this aspect of the invention where a drive rod 51' with a ball end 50' is located between two cones or sockets

67' and 68' in fixed member 52' and wedge-shaped preload lever 53'. Those two cones are shown in FIG.

12 as a fixed cone 67' formed in the upper surface of fixed member 52*, which are affixed to the object to be moved and a preload cone 68'.

The preload cone 68' is an integral part of wedge-shaped lever 53' connected integrally through a flexure pivot 54' to pivot end 64' which is integral with base 63' eliminating the need for a separate preload bar secured to the base 63'. The flexure pivot 54' permits preload cone 68' in lever 53' to be moved through a small angle about the flexure pivot

54' . Preload actuator 55' is mounted so as to provide a preload force acting to press laterally against the preload lever 53* which drives preload cone 68' down against ball end 50'. The compressive preload is achieved without using a preload tensioning rod and nut 59 since wedge-shaped lever 53' is integral with fixed member 52'. Preload sensor 56' is mounted so as to detect the load on the wedge-shaped lever 53' from actuator 55'. Line 62' connects the electrical output from strain gauge 56' into the input of the preload servo and drive electronics 58' whose electrical output signal is connected to the input of the piezoelectric preload actuator 55'.

To increase the preload force upon the ball joint, the actuator 55' is extended or increases in thickness and to reduce the preload force, the actuator is retracted or made thinner, with the servo 58' having a predetermined level programmed into it electronically. The preload force can be altered at a frequency limited only by the bandwidth of the actuator 55*. Servo 58' has another input 61' from a system controller for the purpose of setting the desired level of preload.

FIG. 13 shows another embodiment analogous to FIG. 12 where like elements have like functions, but the structure for applying force to the cones and the ball is modified.

In this embodiment drive rod 51* ' has a ball end 50' ' located between two cones or sockets 67' ' and

68* ' in fixed member 52'' and preload lever 53 I I r respectively. Fixed cone 67'' is formed in the upper surface of fixed member 52'' which is affixed to the object to be moved. Preload cone 68' ' is formed in the lower surface of preload bar 53 ' ' connected integrally through a flexure pivot 54'' to pivot end 64' ' which is integral with base 63' eliminating the need for a separate preload bar secured to the base 63' '. The flexure pivot 54' ' permits preload cone 68' * in member 53'' to be moved through a small angle about the flexure pivot 54' '. Mounted so as to provide a compressive preload force acting to press down against the preload member

53'' and preload cone 68' ' is a preload actuator

55' '. The compressive preload is achieved using a preload tensioning arm 72 integral with base 63' ' . Preload sensor 56'' is mounted so as to detect the load on the bar 53'' from actuator 55* '.

Line 62' ' connects the electrical output from strain gauge 56' ' into the input of the preload servo and drive electronics 58' ' whose electrical output signal is connected to the input of the piezoelectric preload actuator 55' '. To increase the preload force upon the bearing, the actuator 55* ' is extended or increases in thickness and to reduce the preload force, the actuator is retracted or made thinner, with the servo 58'' having a predetermined level programmed into it electronically. The preload force can be altered at a frequency limited only by the bandwidth of the actuator 55' '. Servo 58* ' has another input 61'' from a system controller for the purpose of setting the desired level of preload.

FIG. 14 shows the electrical schematic diagram of the control system for the stage 11. Three identical velocity servos shown in FIG. 2 are used to move the three capstan drive units Ml, M2, and M3. When the servo control electronics 86 receives a new destination from a host computer 105 on lines

103, a series of velocity values are sent to the velocity servos of FIG. 14 to cause the stage 11 to move to the desired new destination. The closed loop position servo loop gain vs frequency and the maximum values of stage velocity, acceleration and rate of change of acceleration are controlled by stored parameters and software in the servo control electronics 86. The velocity servos can be operated with the position servo loop opened during gaging and initialization operations when the laser beams are not activated. Also, the analog joy stick can be used for manual stage control with the Theta servo holding the Theta of the stage near zero.

Fine actuators such a piezoelectric transducers can be added to each of the three drivebars 16, 26, and 36 when higher positional and angular accuracy is required (less than 1 micrometer and 10 microradians. This provides a coarse and a fine servo combination where the fine servo can have a higher gain bandwidth because it is driving only the mass of the stage and the payload. The mass of the drivebar and the inertia of the motors Ml, M2 and M3 are outside of the fine position servo loop.

An X-Y joystick 107 also provides input to the control electronics 86 for manual control of the position of the x-y-theta stage 11.

The Laser Position Transducer and Servo Control electronics 86 receives the X position signals from the output of the XI axis receiver 45 through cable 104. Control electronics 86 also receives the output of Y-axis receiver 85 through cable 116. Electronics 86 also receives the output of theta receiver 84 and through cable 115. Interferometer 110 employs a pair of light beams directed to target mirror 73 as shown in greater detail in FIG. 1. For the Theta and Y axis measurements by receivers 84 and 85, a pair of interferometers are housed in an integrated structure 79 in accordance with this invention as shown in FIG. 1 and FIG. 15A-C. Laser beams 82 and 83 pass to the Theta receiver 84 and Y-axis receiver 85 respectively from the interferometers in structure 79. The interferometers operate with the target mirror 52 as explained in greater detail elsewhere in connection with FIGS. 1 and 15A-15C where the pair of light beams for each are shown, whereas in FIG. 14 they are shown as single beams 200, 201 respectively for convenience of illustration. The electronics 86 have Xl-error output 87 to the positive input of summing circuit 88 which provides an output to X-l drive amplifier 90 which energizes motor Ml which is shown in FIG. 1. The tachometer feedback 91 is shown on line 91 which connects to the negative input of summing circuit 88 to provide negative feedback.

The electronics 86 also have a Y Error output 92 to the positive input of summing circuit 99 and summing circuit 94. Summing circuit 99 provides an output 100 to Y + Theta drive amplifier 101 which energizes motor M3 which is shown in FIG. 1. The tachometer feedback on line 103 is connected to the negative input of summing circuit 99 to provide negative feedback. The electronics 86 ^' further have a Theta- error output 93 to the negative input of summing circuit 94 and the positive input of summing circuit 99.

Summing circuit 94 provides an output 95 to Y - theta drive amplifier 96 which energizes motor M2 which is shown in FIG. 1. The tachometer feedback 98 is shown on line 98 which connects to the negative input of summing circuit 94 to provide negative feedback.

When pure X motion is desired the host computer loads a new X destination. A series of X velocity commands are given to move the X motor Ml and drive bar 16 until the stage position error is driven to zero at the new X location. While the stage is moving in the X axis, the Y position servo is driving the Y Motors M2 and M3 to actively keep the Y position error near zero. Also during the X move Theta disturbances will occur so the Theta servo actively drives the Y motors M2 and M3 differentially to keep the YAW near zero during the move and to hold it near zero after the move. The Theta servo maintains the Theta (YAW angle) of the stage near zero by electronically superimposing small velocity corrections 93 to the Y velocity commands 92 in summing junctions 94 and 99. The dedicated closed loop Theta servo is constantly compensating for tiny gain differences in amplifiers 96 and 101 and motors M2 and M3, inertia differences in the stage and drive bars 26 and 36 and dynamic friction differences in the bearing pads. The Theta servo also removes the Theta disturbances caused by the X drive bar 16. Since the task of maintaining a small value of Theta is accomplished by a dedicated Theta servo, the Y position servo hardware and software can be identical to the X position servo.

If a Theta value other than zero is desired, the host computer can load a Theta value into the electronics and the Y drive bars 26 and 36 will be pushed and pulled with motors M2 and M3 as required to achieve the new Theta value.

Pure Y motion can be achieved by loading a new Y destination and maintaining the same X position.

Integrated Parallel Dual Plane Mirror Interferometers

FIGS. 15A-15C show a pair of interferometers housed in an integrated structure incorporating an embodiment of parallel, dual or two axis plane mirror interferometers in accordance with this invention. Optical elements of the interferometers of FIGS. 1 and FIGS. 17A-C are bonded onto an optically transparent cubic element 129''. Element 129' ' is composed of two bonded glass elements with a polarizing beam-splitting surface 130 joining the two halves of the cubic element 129*'. In other words element 129'' carries bonded thereon the components of the interferometers in an integral structure. The various optical elements are bonded to the element

129' 'cube by means of a uv cured optical cement or the equivalent.

A beam splitter 120 is bonded to the exterior of element 129*' just below a beam bender 121. A laser beam 76 with combined vertical and horizontal polarization components fl, f2 is supplied to the 50% beam splitter 120 which splits the beam in half. 50% of beam 76 is directed out of splitter 120 to become beam 191 for use with one axis of the x y table. 191 is reflected up to the 90-degree beam bender 121 from which it is directed along beam path 192 into the interior of transparent element 129''. As the fl, f2 components of beam 192 from bender 121 encounters beam splitting surface 130 of element 129*', the f2 vertically polarized portion continues as f2 beam 193 to a retroreflector, cube corner 124. The

(horizontally moving) f2 beam 193 is reflected up and over and back at right angles across cube corner 124 and is reflected back along path 183 from the opposite side of cube corner 124 through surface 130 and passes straight along path 83 out of element

129' ', as the f2 component of the output beam 83 to receiver 85. At the same time as the above vertically polarized f2 portion (beam 193) of beam 192 from bender 121 passes through the polarizing beam-splitting surface 130 of element 129' ', e.g. the horizontally polarized fl portion of the beam from bender 121 is reflected down (along beam path 78) through a 1/4 wave plate 123. Plate 123 is bonded to the lower surface of element 129''. The beam 78 passes further down along beam path 78 until it reaches the mirror 52 (shown in FIGS. 1 and 14) , and the beam 78 is reflected back up by mirror 52 at frequency fl +/- delta f along the same beam path 78 through 1/4 wave plate 123 and surface 130 to cube corner 127 where it is doubly reflected (in cube corner 127) over to the right and back down along beam path 77 (parallel to beam path 78) through surface 130 and 1/4 wave plate 123 as beam fl +/- delta f to an adjacent location on the mirror 52 until it is reflected back up as beam fl +/- 2delta f a second time through 1/4 wave plate 123 to beam splitting surface 130 where it is reflected because of its four 1/4 wavelength changes in polarization caused by two passes through plate 123 to pass out along beam path 83, also, with the two beams f2 and fl +/- 2delta f leading to beam path 83. Those two beams passing along beam path 83 one beam from cube corner 124 and a second beam from surface 130 interfere optically with each other as a function of the position of the stage 11 relative to the position of interferometer element 79, which is relatively fixed in position.

The other half of beam path 76 also composed of components fl and f2 from beam splitter 120 for measuring the position of the second axis passes directly along beam path 176 to beam polarizing beam-splitting surface 130. There the vertically polarized f2 portion of the beam is transmitted to cube corner 125. In cube corner 125, that vertically polarized portion of the beam 176 is reflected up and over back through surface 130 out along beam path 82 to the detector 84 in FIGS. 1 and 14. The remaining, horizontally polarized portion of the beam passing along path 176 for the second axis from beam splitter 120 is reflected by the beam-splitting surface 130 down through 1/4 wave plate 122 along beam path 81.

The beam passes along path 81 to mirror 52 and is reflected therefrom back up beam path 81 through 1/4 wave plate 122 again and through surface 130 as a result of the two trips through 1/4 wave plate 122.

That beam passes through to cube corner 126 where it is doubly reflected over and down along beam path 80 through 1/4 wave plate 122 again, passing further along beam path 80 (parallel to beam path 81) to mirror 52 and beam fl+/- 2delta f passes back up along beam path 80 to the polarizing beam-splitting surface 130 where as a result of the effects of 1/4 wave plate 122 it is reflected out along beam path 82 to the detector 84 with the latter beam interfering with the beam which took the other beam route from cube corner 125 to beam path 82 through cube 126.

FIGS. 16A-16C show modifications of the interferometers of FIGS. 15A-15C. In general the operation is the same except that the 50% beam splitter has been moved up to the output of path 82.

The beam on path 82 contains the recombined vertical and horizontally polarized components. The beam 191 reflected upwardly passes through half wave plate 128 to rotate the combined beams 90 degrees and therefrom into the 90 degree beam bender 121 which sends the recombined fl and f2 components of beam 191 through the second integrated interferometer. If component fl was the reference component in beam 76, it will be the measurement component in beam 191, and vice versa. This has the effect of reversing the sign of the detected motion thereby allowing the second axis to be insensitive to translation since as the first axis increments by +df the second axis will decrement by -df giving a net of 0. This assumes that the mirror moves exactly the same amount for both axes.

If it does not then the net will not = 0 and therefore the stage must have rotated to produce the different paths. Therefore, we have an optical signal for Theta.

FIGS. 17A-17C show further modifications of the interferometers of FIGS. 15A-15C. In this case, a 33% beam splitter 140 precedes the 50% beam splitter 120. There are additional cube corners 154 and 155 and an additional 90 degree beam bender 150, as well as quarter wave plate 162 which all combine to measure pitch as well as position and yaw, measured by the prior systems.

Industrial Applicability This x-y positioning system is suitable for use by manufacturers and users of E-beam systems or similar kinds of systems in the semiconductor manufacturing industry. This x-y table positioning system is designed for use with an E-Beam system employed for exposure of lithographic masks for use for semiconductor manufacturing. These drive tables provide improved manufacturing tolerances well below those possible or required in the past.

Claims

What is claimed is:

1. A positioning stage comprising: a) a base; b) a stage plate slideably supported on said base, c) at least three linear drive means rotatably engaging both said stage plate and rotatably engaging said base for moving said stage plate on a path with at least three separate drive displacements along the rectilinearly disposed x and y axes and rotation of said plate on said base substantially parallel to the surface of said base, said plate moving upon the surface of said base, whereby combined x, y rectilinear and rotary motions with respect to said base can be achieved.

2. The stage of claim 1 wherein a) a first one of said linear drive means is directed in a first direction substantially along said x axis, b) a second one of said linear drive means is directed substantially along said y axis transverse to the direction of said first linear drive means, and c) a third one of said linear drive means is substantially parallel to one of said first and second linear drive means for providing rotation through said angle theta, whereby displacement of said stage can be provided in the x, y and rotational directions of motion relative to said base as said three linear drive means independently operate to -30- translate said stage through linear and/or rotary motion with respect to said base.

3. The stage of claim 1 including a) each of drive means on said base being connected to said stage plate for moving said stage plate as a function of its force, b) said drive means including a motor and a capstan driven by said motor, c) a drivebar and pressure means, d) said capstan and said pressure means cooperating to drive said drivebar in response to said motor, e) said drivebars being pivotally mounted on said stage plate and frictionally engaged with said capstan by said pressure means; whereby rotation of said capstan moves said stage plate along a path as a function of said at least three displacements provided by said independent drive means.

The stage of claim 2 including a) each of drive means on said base being connected to said stage plate for moving said stage plate as a unction of its force, b) said drive means including a motor and a capstan driven by said motor, c) a drivebar and pressure means, d) said capstan and said pressure means cooperating to drive said drivebar in response to said motor, e) said drivebars being pivotally mounted on said stage plate and frictionally engaged with said capstan by said pressure means; whereby rotation of said capstan moves said stage plate along a path as a function of said at least three displacements provided by said independent drive means.

5. A positioning system for a stage comprising: a) a base; b) a stage plate slideably supported on said base, c) at least three linear drive means rotatably engaging said stage plate and rotatably engaging said base for moving said stage plate on a path with at least three separate drive displacements motions upon the surface of said base for moving said plate upon the surface of said base, d) means for detecting the displacement of each of said linear drive means, e) control electronics means for providing control signals having inputs connected to said means for detecting and having outputs connected to said linear drive means. whereby combined rectilinear and rotary motion with respect to said base can be achieved.

6. A positioning system in accordance with claim 5 including an interferometer system including an integrated structure including a plurality of interferometer optical systems on a unitary element said system including integrated, parallel, dual two axis plane mirror interferometers in said unitary element.

-32- A positioning system in accordance with claim 5 wherein said means for detecting comprises an interferometer system including an integrated structure including a plurality of interferometer optical systems on a unitary element (129 ' ' ) said element structure (129'') comprising optically transparent material divided into halves and having a plurality of planar external surfaces and a composed of two bonded elements with a polarizing beam-splitting surface (130) at the junction between said two bonded elements, a plurality of optical elements secured to said structure, said elements combining to form at least a pair of interferometers aligned to be directed in parallel at a single retroflecting mirror on a surface whose linear and angular position is to be monitored, a first side of said structure including secured thereto a beam bender (121) and a beam splitter (120) for receiving an input laser beam to be employed by said interferometers having a first optical output path directed into said structure (129'*) and a second optical output directed upwardly towards said beam bender (121) positioned to receive a portion of the beam split from said beam splitter and directing said portion towards said polarizing beam splitting surface (130) ,

first, second, third and fourth parallel retroreflectors (124, 125 and 126, 127) with doubly reflecting surfaces for reflecting a beam parallel to its original path, said first and -33- second retroreflectors (124, 125) being secured to the opposite side of said structure from said first side of said structure (129'') with said second one (125) thereof being aligned with said optical output path of said beam splitter (120) , and said first one (124) thereof being aligned with the optical output path of said beam bender

(121) , and said polarizing beam splitting surface (130) being located between said first side and said second side whereby beams reflected from said beam splitting surface (130) are directed through a third side of said structure (129'*) with a first and a second quarter wave element secured to said third side in alignment with the deflection of beams from said bender (121) and said splitter (120) reflected from said splitting surface (130) respectively to said mirror,

a fourth surface of said structure (129**) located opposite from said third surface, to said third retroreflector (126) and said fourth retroreflector (127) , said third and fourth retroreflectors (126, 127) having their axes aligned with the axes of respective beams reflected from said beam splitting surface (130) and returning back up through said beam splitting surface (130 and passing therethrough to pass through said structure (129*') to reach said third and fourth retroreflectors (126 and 127) respectively, for reflection back down to said mirror respectively for reflection therefrom respectively back to said beam splitting surface (130) along parallel paths, for reflection back up to said beam splitting -34- surface for reflection laterally through said first side of said structure (129 * ') on the same beam path as the reflections from said first and second retroreflectors respectively.

A positioning system in accordance with claim 6 with said polarizing beam-splitting surface (130) of said structure (129'') being positioned in a plane oriented diagonally with respect to the beams from said beam splitter (120) and said beam bender (121) , through the polarizing beam-splitting surface (130) of structure (129' ') , wherein the horizontally polarized portion of the beam being reflected down through a 1/4 wave plate (123) along a beam path (78) to said mirror (52) , and it is reflected back to retroreflector (127) where it is doubly reflected in retroreflector (127) over and down along beam path (77) (parallel to beam path 78) to the mirror (52) until it is reflected back up to beam splitting surface (130) where it is reflected because of its polarization caused by two passes through plate (123) to pass out along beam path (83) , also, with the two beam paths leading to beam path (83) interfering with each other as a function of the distance of movement of the stage (11) ,

the other half of beam path (76) from beam splitter (120) for the second axis passes directly to beam polarizing beam-splitting surface (130) where the vertically polarized portion of the beam is partially transmitted to retroreflector (125) ; that vertically polarized -35- portion of the beam is reflected back from retroreflector (125 out along beam path 82 to the detector 84.

the remaining, horizontally polarized portion of beam for the second axis from beam splitter (120) is partially reflected by the beam-splitting surface (130) down along beam path (81) to mirror (52) and back up beam path (81) and through to retroreflector (126) where it is doubly reflected over and down along beam path (80) to mirror (52) and back up along beam path (80) to the polarizing beam-splitting surface (130) where it is reflected out along beam path (82) to the detector (84) with the latter waves interfering with the waves which took the other beam route to beam path (82) through retroreflector (126) .

A positioning stage in accordance with claim 1 including in each of said linear drive means a ball-type pivot joint with dynamic preload adjustment comprising a) a first member terminating in a ball for said joint, b) a pair of confronting bearing elements each including a bearing surface for one side of said ball, said ball being assembled between said bearing surfaces of said bearing elements, c) at least one of said bearing elements being connected to a preload actuator, d) a preload sensor attached to at least one of said bearing elements. e) servo control means having an input and an output, said input being connected to the output of said preload sensor, and said output being connected to said preload actuator, whereby said preload actuator is dynamically preloaded by operation of said servo control means in response to said preload sensor.

10. A stage in accordance with claim 9 wherein said bearing surfaces are conical.

11. A stage in accordance with claim 9 with said bearing elements including a relatively fixed bearing element and a relatively movable element,

said relatively movable element including a pivot end, said pivot end being secured to said fixed bearing element and a main body end including said bearing surface, said main body end being joined to said pivot end by a flexible section of said movable element, said flexible section providing flexure of said movable element between said main body end and said pivot end.

12. A stage in accordance with claim 11 wherein said bearing surfaces are conical.

13. A stage in accordance with claim 9 with said bearing elements including a relatively fixed bearing element and a relatively movable element, said relatively movable element including a pivot end, said pivot end being integral with said fixed bearing element and a main body end including said bearing surface,

said main body end being joined to said pivot end by a flexible section of said movable element,

said flexible section providing flexure of said movable element between said main body end and said pivot end.

14. A stage in accordance with claim 13 wherein said bearing surfaces are conical.

15. A positioning system in accordance with claim 5 wherein said control electronics means includes means for adjusting the angle of rotation about an axis to a predetermined value.

16. A system in accordance with claim 15 wherein said control electronics adjusts the YAW angle about the 2 axis to near zero.